391
FXUS02 KWBC 011900
PMDEPD

Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
300 PM EDT Mon Sep 1 2025

Valid 12Z Thu Sep 04 2025 - 12Z Mon Sep 08 2025


...Overview...

A deep upper low atop Lake Superior and vicinity as the medium
range period begins Thursday will anchor troughing and resulting
below normal temperatures across the Plains, Midwest, and Great
Lakes/Ohio Valley. Rain is likely ahead of the trough axis,
including some moderate to heavy rain totals possible in the
central/northern Appalachians and interior Northeast later in the
week. Meanwhile in the West, upper ridging is likely to continue
and bring above normal temperatures to the Northwest before a
likely cooling trend by the weekend. Some monsoonal/tropical
moisture may increase late week into the weekend across the West
and southern Plains.


...Guidance/Predictability Assessment...

Model guidance remains in good agreement with the amplified
synoptic scale pattern anchored by the ridge in the West and trough
in the East, with relatively more spread in the details. The
primary upper low`s position continues to converge in deterministic
models and means, and models are also more agreeable now in
dropping a shortwave south through the north-central U.S. on the
western side of the low. The upper low center is forecast to drift
northeastward into the weekend and gradually lift troughing out of
at least southern parts of the Lower 48. However, there may be
opportunities for reinforcing shortwaves into early next week per
some recent model runs (like the ECMWF).

Farther west, there is more spread with the general pattern behind
the Intermountain West ridge. Pacific troughing moves toward the
West Coast at varying speeds, with a lead/weaker shortwave into the
PacNW around Saturday as a northeastern Pacific upper low wraps
south-southwestward to eventually loop back eastward into
California around next Monday. Farther south, NHC is monitoring
potential development just off the Mexican coast, that may lift
northwestward then perhaps turn northward/northeastward across Baja
CA. There remains a large amount of spread as at least several
models do not show such a turn northward/northeastward. Please see
the NHC website for the most recent information.

The WPC forecast was based on a blend of the 00Z-06Z guidance as
the GFS/ECMWF/Canadian/UKMET were clustered fairly well through day
5. Thereafter, with uncertainty in the flow around the upper low
lifting through SE Canada and the incoming trough into the west,
trended toward more ensemble mean weighting.


...Weather/Hazards Highlights...

Convection is likely ahead of the cold front moving through the
northeastern quadrant of the U.S. ahead of the upper low. The front
slowing and tapping into some Atlantic moisture, possibly
influenced by a coastal system, could lead to localized flash
flooding concerns. For the Day 4/Thursday ERO, a Marginal Risk is
delineated across the central to northern Appalachians and
surrounding areas for isolated flooding concerns. While antecedent
conditions are generally dry, heavy rain can sometimes quickly run
off over dry soils. Orographic enhancement may play a role in
producing heavier rain totals as well. The front pushing east into
Friday will shift the convective potential eastward -- a Marginal
Risk is in place for the interior Northeast for the Day 5 ERO,
before rain pulls away Saturday. Some rain may wrap around the
Midwest/Upper Great Lakes late week as upper energy moves through,
and the cool westerly flow atop the Great Lakes could lead to lake
effect rain showers into the weekend.

Monsoonal moisture is forecast to flow into the southwestern U.S.
underneath the mean upper ridge, promoting scattered showers and
thunderstorms. Moisture anomalies are highest into parts of
California and Nevada on Thursday and gradually expanding eastward
into late week. Marginal Risks are in place for eastern California
into the central Great Basin and Southwest on Day 4/Thursday, and
covering much of the Intermountain West farther north and east too
on Day 5/Friday. There is low deterministic QPF over the northern
areas despite favorable conditions, but even the QPF increases late
week into the weekend. The track of an Eastern Pacific tropical
system that is likely to form (see the NHC outlooks) could also
affect the moisture levels in the West, though its path/intensity
is quite uncertain. Rain chances are currently forecast to expand
even farther into the Intermountain West by the weekend. Convection
looks to make its way into the southern Plains at times near a
wavy/lingering frontal boundary, but with plenty of spread in the
details (again related, in part, to any tropical moisture influx).

Meanwhile, frontal boundaries are forecast to meander over Florida
in an unstable environment with above normal moisture, allowing
for localized flash flood potential with urban areas most
vulnerable. Most model guidance then limits potential for heavy
rainfall to south of I-4 into Day 4/Thursday as the front
approaches from the west. South Florida in particular could see
some lingering moisture astride the wavy frontal boundary late this
week into the weekend which would support additional diurnal
convection. Thus, WPC shows a Marginal Risk in the Day 4 ERO across
central to southern Florida and a Marginal limited to South
Florida on Day 5/Friday.

The large trough covering the central and eastern U.S. for the
latter part of this week will promote well below average
temperatures behind multiple cold fronts. Lows are forecast to be
generally around 10-15 degrees below average for Thursday through
the weekend across much of the Plains, Midwest, and Great Lakes
south into the Ohio/Tennessee Valleys. Lows reaching the 30s may
cause concerns for frost around Lake Superior, depending on wind
and cloud cover. Highs could be even more anomalous on Thursday and
Friday across the Upper Midwest/Great Lakes -- around 15-25
degrees below normal. This could set daily records for low maximum
temperatures, as highs only reach the 50s in some locations of the
Upper Midwest and Great Lakes, with even some possible highs in the
40s in the U.P. of Michigan. Temperatures are forecast to
gradually warm closer to normal into early next week as the colder
air mass moves east and moderates as the upper trough lifts out.
Still, temperatures may not quite get back to typical early
September values. On the other hand, the West can expect warm to
hot conditions under the upper ridge continuing into late week.
Temperatures will be most anomalous in the Northwest, with upper
90s and possibly reaching 100F east of the Cascades. Record high
minimum and maximum temperatures are possible but may be more
extensive on Wednesday (now in the short range). HeatRisk shows
Major to locally Extreme conditions in the northern Great Basin
through Thursday, with temperatures decreasing after that.


Fracasso/Tate


Additional 3-7 Day Hazard information can be found on the WPC
medium range hazards outlook chart at:
https://www.wpc.ncep.noaa.gov/threats/threats.php

WPC medium range 500mb heights, surface systems, weather grids,
quantitative precipitation forecast (QPF), excessive rainfall
outlook (ERO), winter weather outlook (WWO) probabilities, heat
indices, and Key Messages can be accessed from:

https://www.wpc.ncep.noaa.gov/medr/5dayfcst500_wbg.gif
https://www.wpc.ncep.noaa.gov/medr/5dayfcst_wbg_conus.gif
https://www.wpc.ncep.noaa.gov/5km_grids/5km_gridsbody.html
https://www.wpc.ncep.noaa.gov/qpf/day4-7.shtml
https://www.wpc.ncep.noaa.gov/#page=ero
https://www.wpc.ncep.noaa.gov/wwd/pwpf_d47/pwpf_medr.php?day=4
https://www.wpc.ncep.noaa.gov/heat_index.shtml
https://www.wpc.ncep.noaa.gov/#page=ovw




$$